In conducting a monitoring operation by using images, additional information (a text, an image, a voice or the like) is sometimes superimposed on a monitoring camera image for realizing an efficient monitoring operation. In, for example, a maintenance/management operation for a plant, information is desired to be superimposed on a monitoring camera image so that instructions for work procedures or things to note can be smoothly checked.
In the case where information is superimposedly displayed on a monitoring camera image, it is necessary to display the information in an appropriate position in a screen in accordance with use of the information. Accordingly, it is necessary to specify what kind of video is currently displayed in a screen. As a method for specifying a current video, a sensor-based positioning method in which various sensors (such as a magnetic sensor, a gyro sensor, an infrared sensor and a GPS) are used for obtaining the position and the attitude of a camera is known.
PTL 1 mentioned below describes a graphic user interface generating apparatus that changes the content or shape of information displayed in a display field region by using attitude determining means for determining the attitude of a camera by using a compass and position determining means for determining the position of the camera on the basis of the latitude and the longitude of the camera. When this technique is employed, information of a message and the like may be made to correspond to a position on an arbitrary real space, so as to be displayed in a display device.
PTL 2 mentioned below describes a method for generating a synthetic image, in which a three-dimensional virtual matter is superimposed on a camera image, on the basis of a three-dimensional space coordinate calculated by using magnetic field generating means and magnetic field detecting means.
On the other hand, a vision-based positioning method for obtaining the position and the attitude of a camera by analyzing a video obtained from the camera without using sensor information is known. As an example of the vision-based positioning method, a method using a marker of a specific pattern whose image can be easily recognized is known.
PTL 3 mentioned below describes a method for recognizing a given image pattern so as to draw information in accordance with the image pattern to be superimposed on an image obtained from a camera.
NPL 1 mentioned below describes a method for superimposing information on a camera image by estimating the position and the attitude of a camera without using a specific marker but using a characteristic point or line detected in an image obtained from the camera.
In these methods, the attitude and the position of a camera are determined on the assumption that the installation position of the camera is changed. If the camera is fixed however, a simpler method may be employed.
PTL 4 mentioned below describes a method for determining a position for displaying additional information on a camera image on the basis of information on the angle of a camera mount and a zoom operation. In this document, an arrangement position of an outer frame of explanatory characters to be superimposed is found so that the sum of luminance differences among pixels adjacent to one another in a region surrounded by the outer frame of the explanatory characters may be minimized, so as to prevent a subject to be picked up by the camera from overlapping the explanatory characters.
PTL 5 mentioned below describes a method for displaying a synthetic image by using camera control information. In this document, it is determined in accordance with a zoom magnification whether or not additional information is displayed.